Ericsson 500 Switch
This is a Ericsson 500 switch used in the automatic phone systems. The switching contact part is missing, so there is only the mechanical part left. First this type of switch in operation in 1923.

This switch is so called turn and push type selector.
It working is based on "register" that is storing the selected numbers (current pulses) and then controlling the working of the switch.
This is different from the Siemens type swithc's which are working in
synchronous to phones number selector.
Power for working is taken from electric motor via this relay. It has three positions: free, forward and backward (pulling the gear up or down to the contact to gears of main shaft).

Then the selector has two other relays that makes
the selector first rotate to correct position (25 positions) and then push the selector out (21 positions).
This picture shows the selector head (other side has one more contact, so totally three).

The selector has a wire grid three wires for each position for turning and for pushing so totally there is
25 times 21 times 3 wires in grid.
Wires are made from bare bronze and their are detached to stand where several selectors can be pushed in.

Two of the wires are used for connecting call and one is reserved for notification purposes.
So one selector can connect to 500 lines (plus one position in every push is for control purposes).
This picture taken from Finnish book (1943) explaining the working of phone system automatic switches.
It shows the principle of automatic switching system made with Ericsson turn and push selectors.

The boxed area in the middle is register that stores the user entered decade number series and converts them to fit the blocks and lines the system has (each selector has 25 positions for turning and 20 positions for pushing).